Yes actually you are correct. After I read this answer I tested it on my personal site by adding the tags around some nonsense words. Not only did Google index the pages with the nonsense words making it into their cache of the pages, but my site ranks for those nonsense words. So while it would be awesome if Googlebot honored those tags, they only work for the Google search appliance!

Maybe in GA, just under where you got that chart from, click "Source" and then plot out the different search engines to validate where that extra traffic is coming from. My assumption if you are seeing all the keywords rise and not sudden traffic from one or a few new keywords is that maybe you just started ranking in one engine across the board? Like maybe all the steady traffic you were seeing before was all from Bing but suddenly Google started blessing the site? I know that doesn't answer your question but at least will eliminate some of the possibilities.

Also, the sources you mention for looking for new backlinks take a while to update (like OSE). So new links wouldn't be showing up in there yet. You might want to look in your traffic sources > referrers report in GA to see if there are some new links in there that you didn't know about before. Or if the total number of unique referrers has increase comparing two time frames? Might be a needle in a haystack but just one more place to look.

Good luck!

Have you tried looking in any of the other link graph tools like ahrefs.com or majestic SEO?